In commonly owned U.S. Pat. No. 3,584,294 there has been disclosed a system of this type in which an incoming a-c signal, after passing through a test pad or over a path independent thereof, is fed to a heterodyning stage including a mixer connected to a local oscillator for producing a beat-frequency oscillation which traverses a multistage digital attenuator controlled by a reversible binary counter. The latter is stepped at a fast or a slow rate by a respective pulse generator of fixed cadence. The counter stops when the attenuated signal matches a reference signal, the amount of attenuation required for the match being a measure of the original signal amplitude and being ascertainable from an analog indicator receiving the count through a digital/analog converter.